1. Field of the Invention
The present invention relates generally to an optical head. In particular, the present invention relates to an optical head having a beam-shaping prism for shaping a beam.
2. Description of the Related Art
FIG. 16A is a front view schematically showing the configuration of a conventional optical head 90, and FIG. 16B is a plan view of the same. The optical head 90 has a semiconductor laser source 5. The semiconductor laser source 5 emits a beam to a polarization beam splitter 6. The beam emitted from the semiconductor laser source 5 passes through the polarization beam splitter 6 to be incident on a collimator lens 4. The collimator lens 4 converts the incident beam into a parallel beam and then emits the parallel beam to a beam-shaping prism 91.
The beam-shaping prism 91 has an entrance surface to which the parallel beam that has been converted by the collimator lens 4 enters and an emission surface from which the parallel beam that has been shaped by the beam-shaping prism 91 is emitted to a hologram 13. The entrance surface and the emission surface are formed so as not to be parallel with each other. The beam-shaping prism 91 shapes the parallel beam incident on its entrance surface so that the width thereof is expanded by a factor of 2.55. The parallel beam thus shaped is then emitted from the emission surface of the beam-shaping prism 91 to the hologram 13.
The parallel beam emitted from the emission surface of the beam-shaping prism 91 passes through the hologram 13 and a quarter-wave plate (not shown) and then is incident on an objective lens 3. The objective lens 3 converges the incident parallel beam on an optical disk 19.
The parallel beam is reflected by the optical disk 19 and passes through the objective lens 3 and the quarter-wave plate (not shown). After passing through the quarter-wave plate, the parallel beam becomes a linearly polarized light orthogonal to an optical path along which the beam emitted from the semiconductor laser source 5 travels toward the optical disk 19 (hereinafter, referred to simply as a “forward optical path”). The linearly polarized light then passes through the hologram 13. Because the parallel beam has a polarization plane orthogonal to the forward optical path, it is split into a zeroth-order diffracted beam and first-order diffracted beams after passing through the hologram 13.
The parallel beam that has been split into the zeroth-order diffracted beam and the first-order diffracted beams by the hologram 13 is incident on the beam-shaping prism 91 again. This time, contrary to the case where the beam travels along the forward optical path, the beam-shaping prism 91 shapes the parallel beam so that the width thereof is reduced by a factor of 2.55. The parallel beam thus shaped is then emitted to the collimator lens 4. After passing through the collimator lens 4, the beam is incident on the polarization beam splitter 6.
Because the polarization plane of the beam incident on the polarization beam splitter 6 is orthogonal to the forward optical path, the beam is reflected by the polarization beam splitter 6 to be incident on a detector 92.
FIG. 17 is a schematic view for illustrating a spot position of the beam incident on the detector 92. The detector 92 has a light-receiving region 99 for receiving the zeroth-order diffracted beam contained in the incident beam. The light-receiving region 99 has a substantially square shape and is divided into four regions having a square shape.
On both sides of the light-receiving region 99 in the direction along which the beam is shaped (hereinafter, referred to simply as the “beam-shaping direction”), light-receiving regions 81 and 82 for receiving the first-order diffracted beams contained in the beam incident on the detector 92 are provided, respectively. Each of the light-receiving regions 81 and 82 is divided along the beam-shaping direction so as to provide three regions having a substantially rectangular shape.
The zeroth-order diffracted beam is incident on a spot position indicated by the oval on the light-receiving region 99. On the other hand, the first-order diffracted beams are incident on spot positions indicated by the ovals on the light-receiving regions 81 and 82, respectively.
For example, in the case where the shape of the beam-shaping prism 91 deviates from the intended shape or the position of the beam-shaping prism 91 deviates from the intended position, the spot positions of the first-order diffracted beams may be displaced from the center portions of the light-receiving regions 81 and 82. For example, the spot position of the first-order diffracted beam indicated by the oval on the light-receiving region 81 may be displaced from the center portion of the light-receiving region 81 toward the right side of FIG. 17, and the spot position of the first-order diffracted beam indicated by the oval on the light-receiving region 82 may be displaced from the center portion of the light-receiving region 82 toward the left side of FIG. 17. In this case, by providing the detector 92 in a swingable manner and swinging the detector 92 in a clockwise direction indicated by the arrow A2 in FIG. 17, the spot position of the first-order diffracted beam on the light-receiving region 81 can be moved toward the center portion of the light-receiving region 81, and the spot position of the first-order diffracted beam on the light-receiving region 82 can be moved toward the center portion of the light-receiving region 82.
FIG. 18 is a view schematically showing the configuration of another conventional optical head 90A, and FIG. 19 is a schematic view for illustrating a spot position of the beam incident on a detector 92A provided in the optical head 90A. In FIGS. 18 and 19, the same components as those in the optical head 90 described above with reference to FIGS. 16A, 16B, and 17 bear the same reference numerals, and their detailed explanation thus has been omitted. The optical head 90A differs from the above-described optical head 90 in that it employs the detector 92A in place of the detector 92.
The detector 92A has a light-receiving region 99 for receiving the zeroth-order diffracted beam contained in the incident beam. The light-receiving region 99 has a substantially square shape and is divided into four regions having a square shape.
On both sides of the light-receiving region 99 in the direction perpendicular to the beam-shaping direction, light-receiving regions 81 and 82 for receiving the first-order diffracted beams contained in the beam incident on the detector 92A are provided, respectively. Each of the light-receiving regions 81 and 82 is divided along the beam-shaping direction so as to provide three regions having a substantially rectangular shape.
The zeroth-order diffracted beam is incident on a spot position indicated by the oval on the light-receiving region 99. On the other hand, the first-order diffracted beams are incident on spot positions indicated by ovals on the light-receiving regions 81 and 82, respectively.
In the case where the shape of the beam-shaping prism 91 only slightly deviates from the intended shape and the position of the beam-shaping prism 91 also only slightly deviates from the intended position, the spot positions of the first-order diffracted beams are in the center portions of the light-receiving region 81 and 82, respectively. Therefore, it is possible to obtain a good detection signal based on the first-order diffracted beams.
However, for example, in the case where the shape of the beam-shaping prism 91 deviates from the intended shape or the position of the beam-shaping prism 91 deviates from the intended position, the spot positions of the first-order diffracted beams may be displaced from the center portions of the light-receiving regions 81 and 82. For example, the spot position of the first-order diffracted beam indicated by the oval on the light-receiving region 81 may be displaced from the center portion of the light-receiving region 81 toward the light-receiving region 99, and the spot position of the first-order diffracted beam indicated by the oval on the light-receiving region 82 may be displaced from the center portion of the light-receiving region 82 toward the light-receiving region 99.
The light-receiving region 82 for receiving the first-order diffracted beam, the light-receiving region 99 for receiving the zeroth-order diffracted beam, and the light-receiving region 81 for receiving the first-order diffracted beam are arranged along the direction perpendicular to the beam-shaping direction. Accordingly, even if the detector is swung in the manner as described above with reference to FIG. 17, the spot position of the first-order diffracted beam on the light-receiving region 81 is not moved toward the center portion of the light-receiving regions 81, nor the spot position of the first-order diffracted beam on the light-receiving region 82 is not moved toward the center portion of the light-receiving regions 82.
The present invention is intended to solve the above-mentioned conventional problems. It is an object of the present invention to provide an optical head by which a good detection signal can be obtained based on a zeroth-order diffracted beam and first-order diffracted beams.